Deception
Ancient Dark Lord
So GV, a Sith Lord able to reproduce Sith Illusions so real, they could kill, a Sith Lord that was able to almost defeat the Republic via brute strength and a surpassing command of the Dark Side, a Sith Lord that only lost due to betrayal, and was yet still able to escape by blowing up a star with his own force powers. That is what power Naga Sadow demonstrated, so now, if a Ragnos on his deathbed, prevented someone like Sadow from killing him, then i'd say he is helluva lot stronger than Sadow, consider the word "respect" did not register in Sadow's list of vocabulary, he never hesitated to backstab nor betray, so for him to fear a Sith Lord such as Ragnos is him admiting his weaker.
Now consider the mastery of the force Yoda has demonstrated, name what he could do that would even put him on par with the Ancient Sith. Name something ROTS Sidious could do to put him on par with the Ancient Sith. The term "Marka Ragnos" when you google it, you always find, that he is described to have a tremendous phyiscal strength and a surpassing command in the dark side of the force, this implies power greater than any other.
Might i add, Sidious is the greatest, and not the most powerful Sithlord, he accomplished for the Sith, that no other Dark Lord had done.
Using more of a scientific term here, is the fact that the Ancient Sith were able to their heavy sith swords with great speed, this requires extreme phyiscal power, consider that an average sword is 40-60kg ( Atleast in some places ) back in the medieval ages. Now consider the size and weight of these Sith Swords, and finally know that the Ancient Sith were able to wield them to same ability that a Jedi wields a lightsaber. This speaks great power, for even the most average Sith Lord in the GAOTS. Now, Kressh was able to break these swords with his bare hand, this indeed implies his great phyiscal prowess and consider Sadow was able to stalemate him, he was no less weaker phyiscally. Yet both these Sith feared the hated half blood Marka Ragnos.
In terms of phyiscal strength the Ancients surpass any of the succeeders, and in knowledge and power of the force, they surpass any other known force users.